This invention relates generally to methods of manufacturing multilayer circuit board and multichip modules. More particularly, this invention relates to new and improved methods of manufacturing multilayer circuits wherein interconnections between circuit layers may be accomplished in a single fusion bonding lamination step utilizing fusible conductive material. In a preferred embodiment, this fusion bonding is accomplished without the need for intermediate bonding plys.
Multilayer circuits are well known and comprise a plurality of stacked substrate/circuit trace assemblies with interconnections between selected locations on the spaced circuit traces. These circuits are often configured as stripline or microstrip circuits and find particular utility in microwave antenna and other microwave applications. These circuits are also used in the manufacture of miltichip modules. Conventional manufacturing techniques for multilayer circuits sometimes require multiple bonding cycles to make the completed assembly. Once the substrates have been bonded together, via holes are drilled completely through the bonded assembly and plated for transferring electrical connections between different layers. This limits the circuit density and the number of substrates. The bonding material, which may comprise a bonding film between the substrates, tends to flow out of the assembly under the influence of the high temperature and pressure. As more and more of the bonding material flows out, the pressure is transferred to the plated holes which may collapse and/or be pushed into the substrate which has been softened by the high temperature.
U.S. Pat. No. 4,788,766 attempts to overcome these problems. This prior patent discloses a method wherein a multilayer assembly is made up of a number of individual circuit boards and each board has a substrate on which a first conductive layer is formed on the opposite surface. The substrate is a dielectric material which insulates the conductive layers. Via holes are formed through the first conductive layer, the substrate and the second conductive layer at various locations. An outer conductive material, such as copper, is applied over the first and second conductive bonding layers and onto the side walls of the holes. A conductive bonding material is then deposited onto the outer conductive material in the areas around the holes. Once the individual boards have been fabricated, they are stacked in a predetermined order and orientation with a suitable low temperature dielectric bonding ply (meaning that the bonding ply has a lower softening temperature than the circuit substrate material) positioned between each pair of layers. The dielectric bonding ply requires registered apertures therethrough which correspond to areas where the conductive layer of one substrate is to make an electrically conductive connection with the conductive layer of an adjacent substrate. Thus, the dielectric bonding ply integrally bonds adjacent boards together while providing electrical isolation and/or electrical connections between conductive layers of the different boards. The assembly of boards is then subjected to a cycle of heat and pressure to effect a bond between the various board layers.
While the method of U.S. Pat. 4,788,766 overcomes some of the problems in the prior art, this prior method has certain disadvantages including the requirement for a substrate which has a melting temperature above the melting temperature of the bonding ply. In other words, the prior patent necessitates the use of a low temperature bond ply which limits the thermal rating of the multi-layer circuit. In addition, this prior method necessitates registered apertures in the bonding ply (leading to alignment problems) and is limited to multilayer circuits having plated through holes.